How to build a greenhouse

Save money and garden all year long by building a greenhouse.

DIY: You can easily build your own greenhouse over the course of a weekend. (Photo: Joi/Flickr)

Do you dream of having your very own dedicated space for growing flowers, plants, and even your own food year-round?

If you think a greenhouse is out of reach, think again: you can construct a simple structure, at an affordable price, in a weekend’s time. All you need is space in your yard, a few basic supplies, and some elbow grease, and you’ll have a functional greenhouse in no time. Don’t have space? There are options for you, too, whether it’s a window-mounted box or a slightly larger, but temporary, cold-frame. Want to go all out? With the right supplies and a good construction plan — and a healthy DIY attitude — you can create a traditional wood-framed, glass-paneled greenhouse, but with salvaged materials and an alternative approach that will be kinder to your wallet.

Why build your own greenhouse?

Building your own greenhouse is smart for many reasons. To name a few:

1. You can garden all year long. If you live in an area with a harsh climate, a greenhouse will let you enjoy your passion anytime you want. A properly maintained backyard greenhouse will protect plants from the elements — snow, strong winds, heavy rains, hail — as well as provide a controlled environment where plants that might struggle outdoors can thrive. Even if the weather conditions in your area are ideal for gardening, a greenhouse will enable you to get a head start, planting vegetables and other plants earlier in the season.

2. You can save money. By growing your own food, you cut out the shipping costs that inflate the prices of store-bought produce. (And no shipping means homegrown food is better for the environment, too). You can grow fruits, vegetables, and plants that might cost more at stores when they are not in season. And you save money by building your own backyard greenhouse rather than buying a prefab one or using a kit.

3. You can be in control of your food. With a backyard greenhouse, you decide what you want to grow and when. You decide what goes into the food you grow, so you know exactly what you’re eating.

4. You can work with your space. A premade greenhouse won't be designed specifically for your space. When you build your own greenhouse, you determine what best suits your needs and your land.

What you need to know before you start

With so many options for greenhouse structures, you'll first want to determine what you need and what you can build. What do you want to grow? How much do you want to grow? How do you want to use the greenhouse? What is your region’s climate? How much do you have to spend? How much space do you have? What is allowable in your area per municipal codes?

Any greenhouse has four basic requirements:

Warmth: a heating system (mechanical or natural)

Moisture: an irrigation system (manual or automated)

Protection: from the elements and pests

Control: of air circulation and temperature (with electrical systems or by manual maintenance)

A heating system is critical for the success of a greenhouse. Common sources of heat are electrical heaters and gas-, oil-, and wood-fueled heating systems (all of which must be vented to the outside). If you want to try heating your greenhouse naturally, try passive solar heat. This system involves a “heat sink,” which stores heat during the day that can be used when the sun goes down. (See “How to Make a Passive Solar Greenhouse.”) How much heat you need depends on what you're growing, how big your structure is, how much of the outside structure is exposed, and what it's made of. Regardless of the type of structure, insulation is key: you might have a state-of-the-art heating system in place, but if your structure isn't airtight, your greenhouse will not be successful.

Finally, determine the best place for your greenhouse. You'll want the spot in your yard that gets the most sun (preferably all-day sun, but good morning sun is acceptable), which is often the south or southeast side of your home. If you build near trees, deciduous trees are best, because they shed their leaves in the fall. Avoid evergreen trees that will block the winter sun.

How to build a PVC greenhouse

A PVC greenhouse is a perfect option if you have a limited budget and a small backyard space (at least 12 feet long by 6 feet wide — anything smaller may make controlling the environment a challenge). This freestanding greenhouse constructed of a wood foundation, a PVC frame, and a plastic film cover is easy to assemble and requires a short list of supplies. Also called a high tunnel, it has a Quonset (semicircular arched) shape and measures 12 feet by 14 feet.

Materials

For this basic PVC greenhouse, you’ll need the following materials:

(16) 10-foot lengths of ¾”, schedule-80 PVC pipe

(6) 4-way, or cross, PVC connectors

(2) 3-way, or tee, PVC connectors

PVC cement to secure joints

(32) ¾” galvanized EMT (electrical metallic tubing) straps

(2) 2”x6”x14’ treated wood boards

(2) 2”x6”x12’ treated wood boards

(4) 2”x4”x7’ treated wood boards

(4) 2”x4”x6’ treated wood boards

(4) 4”x4”x2’ treated wood boards

(2) 2”x4”x3’ treated wood boards

(2) 1”x4”x12’ treated wood boards

Door hinges

Sheet of 24’x20’ 4-mil UV-resistant clear plastic film

Nails, screws, and staples

A saw or pipe cutter

A drill

A stapler

A GFCI outlet (if you'll be using an electrical heating system or a power tool inside)

Optional materials: mesh wire to keep out critters; and a material (such as EMT conduit or rebar) to reinforce the PVC midrib if you live in a snowy climate

Instructions

1. Make sure your foundation is level. You want the wood frame to touch the ground at all points so that water doesn't leak in and heat doesn't leak out — but it should not be positioned below the ground.

2. Build the foundation frame. Before connecting the side boards to the end boards, loosely attach the EMT straps that will hold the PVC ribs in place. Place 4”x4” anchor posts at each corner of the frame for strength.

3. Assemble the PVC frame. Build the midrib, which runs the length of the greenhouse, by connecting seven 22 ½” pieces of PVC with 4-way connectors (use the 3-way connectors for the two ends) and cementing them; then add the vertical ribs by inserting a 10-foot PVC pipe into each of the remaining openings in the connectors. Place the PVC frame over the foundation frame and, with a set of helping hands, bend the vertical ribs to create bows, securing them at the bottom to the foundation with the EMT straps.

4. Add the end frames and the door. Attach two 2”x4”x7’ boards diagonally on each end frame for support. Use the remaining wood boards to frame out the door.

5. Secure the plastic film. With a helper, spread the plastic tightly over the top of the PVC frame and staple it to the foundation, leaving a few inches overlapping the frame, which you can cover with packed soil to create a tighter seal.

6. Cover the floor with gravel or sand.

Tips

1. The plastic film that covers your greenhouse will degrade in a few years and need to be replaced. The ideal method of disposal is recycling, but agricultural plastic recycling is not available everywhere, so consider storing it until you can dispose of it in an environmentally friendly manner.

2. This type of greenhouse is not good for areas that receive high winds or heavy snows. To prevent against collapse under snow, reinforce the greenhouse midrib with EMT conduit or steel rebar.

3. Choose wood for your foundation and door frames that is long-lasting and sustainable. Cedar, cypress, and hemlock are affordable options; redwood is also a good choice but is more expensive.

4. To keep conditions more constant, you can cover your greenhouse with a shade cloth, or locate it near deciduous trees that provide some natural afternoon shade.

Note: While PVC is a cheap and durable material, making it ideal for an economical greenhouse construction, it is not particularly good for the environment. It requires a significant amount of energy to produce, and the process results in toxic chemicals (such as phthalates) being released into the environment. Disposal is also a challenge; because of its particular combination of additives, it is difficult to recycle. If you can spend more and want to be friendlier to the environment, consider alternatives: galvanized steel and EMT conduit are two popular options.

How to make a passive solar greenhouse

A greenhouse is not a greenhouse without heat. Traditional heating systems, such as electric and gas heaters, can do the trick, but if you want to get “free” heat, and cut down on those utility bills, consider passive solar heat. The system involves a “heat sink” that collects and stores heat waves from plantings in the greenhouse throughout the day and then distributes the heat at night. The heat sink can be any of a number of items: a dark-painted tank filled with water, a pile of stones in a wire cage, a wall made of bricks, or a concrete slab on top of a rock bed.

To incorporate a passive-solar heating system into your greenhouse, you’ll just need to determine how big of a heat sink you’ll need and allot a space for it (calculate how much heat you’ll need at http://www.hobby-greenhouse.com/FreeSolar.html). This system is most successful in sunny climates, but it may be wise to have a backup conventional system just in case. To ensure your heat sink can store the heat it collects, make your greenhouse airtight.

If you want a more efficient solar system and have a little cash to spend, try an active solar system, which involves an electrical system that pumps heated air into a storage area.

Other greenhouse options

A PVC greenhouse is just one of many options in the world of DIY greenhouses. If you want something more permanent, but still affordable, consider a wood-and-fiberglass lean-to greenhouse. This attached structure shares a wall with your home, eliminating construction costs of a fully detached greenhouse. And because it is connected to your home, it can benefit from your home’s HVAC system — in the hot summer months, you can open the door between the two buildings to let cooler air into the greenhouse, and in the cold winter months, your greenhouse will benefit from the heated wall it shares with your home.

On the other end of the spectrum is a simple, temporary cold-frame. This low, unheated structure can be built with salvaged lumber and takes up little space in a yard. It is used primarily for starting plantings early in the season, growing seedlings, or hardening off plants grown indoors. The transparent top is removable to allow for air flow, watering, and heat exposure. When the cold-frame has served its purpose and you’ve moved your plants into your outdoor garden, you can store the structure away. Don’t have space for even a small cold-frame? Look into getting a plot in a community garden where you may be able to install one.